Distributed bragg reflector structures in multijunction solar cells

ABSTRACT

A multijunction solar cell and its method of fabrication, having an upper first solar subcell composed of a semiconductor material including aluminum and having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; a first and second DBR structure adjacent to the third solar subcell; and a fourth solar subcell adjacent to the DBR structures and lattice matched with said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the fourth subcell has a direct bandgap of greater than 0.75 eV.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/376,195 filed Dec. 12, 2016.

This application is related to co-pending U.S. patent application Ser.No. 14/660,092 filed Mar. 17, 2015, which is a division of U.S. patentapplication Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No.9,018,521; which was a continuation in part of U.S. patent applicationSer. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to co-pending U.S. patent applicationSer. No. 13/872,663 filed Apr. 29, 2013, now U.S. Pat. No. 10,541,349which was also a continuation-in-part of application Ser. No.12/337,043, filed Dec. 17, 2008.

This application is also related to U.S. patent application Ser. No.14/828,197, filed Aug. 17, 2015.

All of the above related applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication ofsolar cells, and more particularly the design and specification of alattice matched multijunction solar cells adapted for specific spacemissions.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to properly specify andmanufacture. Typical commercial III-V compound semiconductormultijunction solar cells have energy efficiencies that exceed 27% underone sun, air mass 0 (AM0) illumination, whereas even the most efficientsilicon technologies generally reach only about 18% efficiency undercomparable conditions. The higher conversion efficiency of III-Vcompound semiconductor solar cells compared to silicon solar cells is inpart based on the ability to achieve spectral splitting of the incidentradiation through the use of a plurality of photovoltaic regions withdifferent band gap energies, and accumulating the current from each ofthe regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads becomemore sophisticated, and applications anticipated for five, ten, twentyor more years, the power-to-weight ratio and lifetime efficiency of asolar cell becomes increasingly more important, and there is increasinginterest not only the amount of power provided at initial deployment,but over the entire service life of the satellite system, or in terms ofa design specification, the amount of power provided at the “end oflife” (EOL).

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, witheach subcell being designed for photons in a specific wavelength band.After passing through a subcell, the photons that are not absorbed andconverted to electrical energy propagate to the next subcells, wheresuch photons are intended to be captured and converted to electricalenergy.

The energy conversion efficiency of multijunction solar cells isaffected by such factors as the number of subcells, the thickness ofeach subcell, the composition and doping of each active layer in asubcell, and the consequential band structure, electron energy levels,conduction, and absorption of each subcell, as well as the effect of itsexposure to radiation in the ambient environment over time. Theidentification and specification of such design parameters is anon-trivial engineering undertaking, and would vary depending upon thespecific space mission and customer design requirements. Since the poweroutput is a function of both the voltage and the current produced by asubcell, a simplistic view may seek to maximize both parameters in asubcell by increasing a constituent element, or the doping level, toachieve that effect. However, in reality, changing a material parameterthat increases the voltage may result in a decrease in current, andtherefore a lower power output. Such material design parameters areinterdependent and interact in complex and often unpredictable ways, andfor that reason are not “result effective” variables that those skilledin the art confronted with complex design specifications and practicaloperational considerations can easily adjust to optimize performance.Electrical properties such as the short circuit current density(J_(sc)), the open circuit voltage (V_(oc)), and the fill factor (FF),which determine the efficiency and power output of the solar cell, areaffected by the slightest change in such design variables, and as notedabove, to further complicate the calculus, such variables and resultingproperties also vary, in a non-uniform manner, over time (i.e. duringthe operational life of the system).

Another important mechanical or structural consideration in the choiceof semiconductor layers for a solar cell is the desirability of theadjacent layers of semiconductor materials in the solar cell, i.e. eachlayer of crystalline semiconductor material that is deposited and grownto form a solar subcell, have similar crystal lattice constants orparameters.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide increasedphotoconversion efficiency in a multijunction solar cell for spaceapplications by incorporating a plurality of distributed Bragg reflectorstructures between two adjacent subcells in the multijunction solarcell.

It is another object of the present disclosure to provide amultijunction solar cell in which the distributed Bragg reflectorstructures have different wavelength bands of reflectivity.

It is another object of the present disclosure to provide amultijunction solar cell in which the DBR structure or structuresenables a transition in lattice constant between two subcells.

It is another object of the present invention to provide a latticematched four junction solar cell incorporating a plurality of differentDBR structures.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingobjects.

Features of the Invention

All ranges of numerical parameters set forth in this disclosure are tobe understood to encompass any and all subranges or “intermediategeneralizations” subsumed herein. For example, a stated range of “1.0 to2.0 eV” for a band gap value should be considered to include any and allsubranges beginning with a minimum value of 1.0 eV or more and endingwith a maximum value of 2.0 eV or less, e.g., 1.0 to 2.0, or 1.3 to 1.4,or 1.5 to 1.9 eV.

Briefly, and in general terms, the present disclosure provides amultijunction solar cell comprising: a first and a second solar subcelleach having an emitter layer and a base layer forming a photoelectricjunction; a first distributed Bragg reflector (DBR) structure disposedbeneath the base layer of the first solar subcell and composed of aplurality of alternating layers of different semiconductor materialswith discontinuities in their respective indices of refraction andarranged so that light can enter and pass through the first solarsubcell and at least a first portion of which in a first spectralwavelength range [of 840 to 890 nm] can be reflected back into the firstsolar subcell by the DBR structure, and a second portion of which in asecond spectral wavelength range [of 790 to 840 nm] can be transmittedthrough the DBR structure to the second solar subcell disposed beneaththe DBR structure, where the second wavelength range is greater inwavelength than the first wavelength range [wherein the half width valueof reflection of the DBR structure being in a range between 250 nm to350 nm]; and wherein the alternating first and second sublayer have adifferent lattice constant.

In some embodiments, the upper first subcell is composed of indiumgallium aluminum phosphide, with the amount of aluminum being at least20% by mole fraction.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising providing agermanium substrate; growing on the germanium substrate a latticematched sequence of layers of semiconductor material using a metalorganic chemical vapor disposition process to form a plurality ofsubcells including one or more middle subcells and a DBR structuredisposed over and lattice mismatched to the germanium substrate and anupper or top subcell disposed over and lattice matched to the lastmiddle subcell and having a band gap in the range of 2.0 to 2.15 eV.

In another aspect, the present disclosure provides a method offabricating a four junction solar cell comprising an upper first solarsubcell composed of indium gallium aluminum phosphide and having a firstband gap, a second solar subcell adjacent to said first solar subcellincluding an emitter layer composed of indium gallium phosphide oraluminum gallium arsenide, and a base layer composed of aluminum galliumarsenide and having a second band gap smaller than the first band gapand being lattice matched with the upper first solar subcell, a thirdsolar subcell adjacent to said second solar subcell and composed ofindium gallium arsenide and having a third band gap smaller than thesecond band gap and being lattice matched with the second solar subcell;a DBR structure adjacent to the third solar subcell; and a fourth solarsubcell adjacent to said DBR structure and having a fourth band gapsmaller than the third band gap.

In some embodiments, the fourth subcell is germanium.

In some embodiments, the fourth subcell is InGaAs, GaAsSb, InAsP,InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN,InGaBiN, InGaSbBiN.

In some embodiments, the fourth subcell has a band gap of approximately0.67 eV, the third subcell has a band gap of approximately 1.41 eV, thesecond subcell has a band gap in the range of approximately 1.65 to 1.8eV and the upper first subcell has a band gap in the range of 2.0 to 2.2eV.

In some embodiments, the second subcell has a band gap of approximately1.73 eV and the upper first subcell has a band gap of approximately 2.10eV.

In some embodiments, the upper first subcell is composed of indiumgallium aluminum phosphide; the second solar subcell includes an emitterlayer composed of indium gallium phosphide or aluminum gallium arsenide,and a base layer composed of aluminum gallium arsenide; the third solarsubcell is composed of indium gallium arsenide; and the fourth subcellis composed of germanium.

In some embodiments, the first and second distributed Bragg reflector(DBR) structures are disposed adjacent to and between the middle andbottom solar subcells and arranged so that light can enter and passthrough the middle solar subcell and at least a portion of which can bereflected back into the middle solar subcell by the DBR structures.

In some embodiments, the first and second distributed Bragg reflector(DBR) structures are disposed adjacent to and between the second and thethird solar subcells and arranged so that light can enter and passthrough the through the third solar subcell and at least a portion ofwhich can be reflected back into the third solar subcell by the DBRstructures.

In some embodiments, each of the distributed Bragg reflector structuresare composed of a plurality of alternating layers of lattice matchedmaterials with discontinuities in their respective indices ofrefraction.

In some embodiments, at least some of the layers of at least one of thedistributed Bragg reflector structures is composed of a plurality ofalternating layers of different lattice constant.

In some embodiments, at least some of the layers of the distributedBragg reflector structures are composed of a plurality of alternatinglayers having different doping levels and/or different dopant materials.

In some embodiments, at least some of the layers of the distributedBragg reflector structures are composed of a plurality of alternatinglayers of different thicknesses.

In some embodiments, the width of the first spectral wavelength range isbetween 50 and 100 nm.

In some embodiments, the first spectral wavelength range extends between840 and 890 nm.

In some embodiments, the first spectral wavelength range overlaps thesecond wavelength range by less than 10 nm.

In some embodiments, the first and second spectral wavelength rangescorrespond to the spectral absorption band of the first solar subcell.

In some embodiments, the difference in refractive indices betweenalternating layers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength.

In some embodiments, each of the distributed Bragg reflector structuresare composed of a plurality of alternating layers that includes a firstDBR layer composed of an n type or p type Al_(x)Ga_(1-x)As layer, and asecond adjacent DBR layer disposed over the first DBR layer and composedof an n or p type Al_(y)Ga_(1-y)As layer, 0<x<1, 0<y<1, and where y isgreater than x.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1A is a graph representing the band gap of certain binary materialsand their lattice constants;

FIG. 1B is a graph representing the efficiency of two tandem subcells asa function of the band gap of the two subcells;

FIG. 2A is a graph of the band gap versus lattice constant of certainbinary and ternary III-V semiconductors;

FIG. 2B is a graph of the band gap versus lattice constant of certainbinary and ternary III-V semiconductors;

FIG. 3A is a cross-sectional view of a three junction solar cell afterseveral stages of fabrication including the deposition of certainsemiconductor layers on the growth substrate, according to a firstembodiment of the present disclosure;

FIG. 3B is a cross-sectional view of a four junction solar cell afterseveral stages of fabrication including the deposition of certainsemiconductor layers on the growth substrate, according to a secondembodiment of the present disclosure;

FIG. 3C is a cross-sectional view of the solar cell of a four junctionsolar cell after several stages of fabrication including the depositionof certain semiconductor layers on the growth substrate, according to athird embodiment of the present disclosure;

FIG. 4A is a graph of the current density per unit energy versus thephoton energy of the incoming light in a solar cell;

FIG. 4B is a schematic representation of photons of differentwavelengths being absorbed by, or being transmitted through, differentsubcells in a three junction tandem solar cell with a single DBRstructure;

FIG. 5A is a graph of the reflectance of a single distributed Braggreflector (DBR) structure as a function of wavelength;

FIG. 5B is a graph of the reflectance of a first distributed Braggreflector (DBR) structure according to the present disclosure comparedwith that of the structure of FIG. 5A;

FIG. 5C is a graph of the reflectance of a first distributed Braggreflector (DBR) structure according to the present disclosure comparedwith that of the structure of FIG. 5A;

FIG. 6 is a schematic representation of photons of different wavelengthsbeing absorbed by, or being transmitted through, different subcells in asolar cell that includes two distributed Bragg reflector (DBR)structures according to the present disclosure;

FIG. 7A is a graph of the quantum efficiency versus wavelength in athree junction solar cell;

FIG. 7B is a graph of the quantum efficiency versus wavelength in athree junction solar cell after incorporation of a structure in thesolar cell according to the present disclosure; and

FIG. 8 is a graph of the doping profile in the base and emitter layersof a subcell in the solar cell according to the present disclosure.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one elements from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cellwhich is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density J_(sc)through a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refersto a layer of material which is epitaxially grown over anothersemiconductor layer.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell”refers to a solar cell in which the subcells are deposited or grown on asubstrate in a “reverse” sequence such that the higher band gapsubcells, which would normally be the “top” subcells facing the solarradiation in the final deployment configuration, are deposited or grownon a growth substrate prior to depositing or growing the lower band gapsubcells.

“Layer” refers to a relatively planar sheet or thickness ofsemiconductor or other material. The layer may be deposited or grown,e.g., by epitaxial or other techniques.

“Lattice mismatched” refers to two adjacently disposed materials orlayers (with thicknesses of greater than 100 nm) having in-plane latticeconstants of the materials in their fully relaxed state differing fromone another by less than 0.02% in lattice constant. (Applicant expresslyadopts this definition for the purpose of this disclosure, and notesthat this definition is considerably more stringent than that proposed,for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6%lattice constant difference).

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell whichis neither a Top Subcell (as defined herein) nor a Bottom Subcell (asdefined herein).

“Short circuit current (I_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero volts, as represented and measured, for example,in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electronic device operable to convert theenergy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells being substantially identical (i.e.within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in amultijunction solar cell which is closest to the primary light sourcefor the solar cell.

“ZTJ” refers to the product designation of a commercially availableSolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well asinverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with the latticematched or “upright” solar cells of the present disclosure. However,more particularly, the present disclosure is directed to the fabricationof a multijunction lattice matched solar cell with specific DBRstructures grown between subcells.

Prior to discussing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells, and the context of the composition ordeposition of various specific layers in embodiments of the product asspecified and defined by Applicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes, such an“optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and directionand the ultimate solar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the thickness, band gap, doping, or othercharacteristic of the incorporation of that material in a particularsubcell, is not a single “result effective variable” that one skilled inthe art can simply specify and incrementally adjust to a particularlevel and thereby increase the power output and efficiency of a solarcell.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, let alonewhether it can be fabricated in a reproducible high volume manner withinthe manufacturing tolerances and variability inherent in the productionprocess, and necessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact inunpredictable ways, the proper choice of the combination of variablescan produce new and unexpected results, and constitute an “inventivestep”.

The efficiency of a solar cell is not a simple linear algebraic equationas a function of the amount of gallium or aluminum or other element in aparticular layer. The growth of each of the epitaxial layers of a solarcell in a reactor is a non-equilibrium thermodynamic process withdynamically changing spatial and temporal boundary conditions that isnot readily or predictably modeled. The formulation and solution of therelevant simultaneous partial differential equations covering suchprocesses are not within the ambit of those of ordinary skill in the artin the field of solar cell design.

More specifically, the present disclosure intends to provide arelatively simple and reproducible technique that does not employinverted processing associated with inverted metamorphic multijunctionsolar cells, and is suitable for use in a high volume productionenvironment in which various semiconductor layers are grown on a growthsubstrate in an MOCVD reactor, and subsequent processing steps aredefined and selected to minimize any physical damage to the quality ofthe deposited layers, thereby ensuring a relatively high yield ofoperable solar cells meeting specifications at the conclusion of thefabrication processes.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a depositionmethod, such as Molecular Beam Epitaxy (MBE), Organo Metallic VaporPhase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),or other vapor deposition methods for the growth may enable the layersin the monolithic semiconductor structure forming the cell to be grownwith the required thickness, elemental composition, dopant concentrationand grading and conductivity type, and are within the scope of thepresent disclosure.

The present disclosure is in one embodiment directed to a growth processusing a metal organic chemical vapor deposition (MOCVD) process in astandard, commercially available reactor suitable for high volumeproduction. Other embodiments may use other growth technique, such asMBE. More particularly, regardless of the growth technique, the presentdisclosure is directed to the materials and fabrication steps that areparticularly suitable for producing commercially viable multijunctionsolar cells or inverted metamorphic multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

Some comments about MOCVD processes used in one embodiment are in orderhere.

It should be noted that the layers of a certain target composition in asemiconductor structure grown in an MOCVD process are inherentlyphysically different than the layers of an identical target compositiongrown by another process, e.g. Molecular Beam Epitaxy (MBE).

The material quality (i.e., morphology, stoichiometry, number andlocation of lattice traps, impurities, and other lattice defects) of anepitaxial layer in a semiconductor structure is different depending uponthe process used to grow the layer, as well as the process parametersassociated with the growth. MOCVD is inherently a chemical reactionprocess, while MBE is a physical deposition process. The chemicals usedin the MOCVD process are present in the MOCVD reactor and interact withthe wafers in the reactor, and affect the composition, doping, and otherphysical, optical and electrical characteristics of the material. Forexample, the precursor gases used in an MOCVD reactor (e.g. hydrogen)are incorporated into the resulting processed wafer material, and havecertain identifiable electro-optical consequences which are moreadvantageous in certain specific applications of the semiconductorstructure, such as in photoelectric conversion in structures designed assolar cells. Such high order effects of processing technology do resultin relatively minute but actually observable differences in the materialquality grown or deposited according to one process technique comparedto another. Thus, devices fabricated at least in part using an MOCVDreactor or using a MOCVD process have inherent different physicalmaterial characteristics, which may have an advantageous effect over theidentical target material deposited using alternative processes.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1A is a graph representing the band gap of certain binary materialsand their lattice constants. The band gap and lattice constants ofternary materials are located on the lines drawn between typicalassociated binary materials (such as the ternary material AlGaAs beinglocated between the GaAs and AlAs points on the graph, with the band gapof the ternary material lying between 1.42 eV for GaAs and 2.16 eV forAlAs depending upon the relative amount of the individual constituents).Thus, depending upon the desired band gap, the material constituents ofternary materials can be appropriately selected for growth.

FIG. 1B is a graph representing the efficiency of two tandem subcells asa function of the band gap of the two subcells. In particular, it isdepicted to demonstrate that the maximum efficiency of tandemcombination of subcells is not a simple linear function of the band gapof either the top or high band gap subcell, or the lower or low band gapsubcell.

FIG. 2A is an enlargement of a portion of the graph of FIG. 1Aillustrating different compounds of GaInAs and GaInP with differentproportions of gallium and indium, and the location of specificcompounds on the graph.

FIG. 2B is a representation of the theoretical efficiency of a tandemsolar cell in which the band gap of the top solar subcell is plottedalong the y-axis, and the band gap of the adjacent middle solar subcellis plotted along the x-axis graph, with the two ternary compoundsGa_(x)In_(1-x)As and Ga_(y)In_(1-y)P having identical lattice constantsbeing plotted as a straight line.

FIG. 3A illustrates a particular example of an embodiment of a threejunction solar cell 3000 after several stages of fabrication includingthe growth of certain semiconductor layers on the growth substrate up tothe contact layer 322 as provided by the present disclosure.

As shown in the illustrated example of FIG. 3A, the bottom subcell Cincludes a substrate 300 formed of p-type germanium (“Ge”) which alsoserves as a base layer. A back metal contact pad 350 formed on thebottom of base layer 300 provides electrical contact to themultijunction solar cell 400. The bottom subcell C, further includes,for example, a highly doped n-type Ge emitter layer 301, and an n-typeindium gallium arsenide (“InGaAs”) nucleation layer 302. The nucleationlayer is deposited over the base layer, and the emitter layer is formedin the substrate by diffusion of deposits into the Ge substrate, therebyforming the n-type Ge layer 301. Heavily doped p-type aluminum galliumarsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”)tunneling junction layers 303, 304 may be deposited over the nucleationlayer to provide a low resistance pathway between the bottom and middlesubcells.

A first Distributed Bragg reflector (DBR) structure (DBR-2) consistingof layers 305 (specifically, individual layers 305 a through 305 z) arethen grown adjacent to and between the tunnel diode 303, 304 of thebottom subcell C and the second solar subcell B. The DBR layers 305 arearranged so that light can enter and pass through the third solarsubcell B and DBR structure 306 and at least a portion of which can bereflected back into the second solar subcell B by the DBR layers 305. Inthe embodiment depicted in FIG. 3A, the distributed Bragg reflector(DBR) layers 305 are specifically located between the second solarsubcell B/DBR structure 306 and tunnel diode layers 304, 303; in otherembodiments, the distributed Bragg reflector (DBR) layers may be locatedbetween tunnel diode layers 304/303 and buffer layer 302.

A second Distributed Bragg reflector (DBR) structure (DBR-1) consistingof layers 306 (specifically, 306 a through 306 z) being compositionallyand optically different from DBR structure DBR-1, are then grownadjacent to and between the DBR-2 structure and the second solar subcellB. The DBR layers 306 are arranged so that light can enter and passthrough the third solar subcell B and at least a portion of which can bereflected back into the third solar subcell B by the DBR layers 306. Inthe embodiment depicted in FIG. 3A, the distributed Bragg reflector(DBR) layers 306 are specifically located between the second solarsubcell B and tunnel diode layers 304, 303; in other embodiments, thedistributed Bragg reflector (DBR) layers 306 may be located betweentunnel diode layers 304/303 and DBR-2 structure.

For some embodiments, distributed Bragg reflector (DBR) layers 305 and306 can be composed of a plurality of alternating layers 305 a through305 z and 306 a through 306 z, respectively, of lattice matchedmaterials with discontinuities in their respective indices ofrefraction. For certain embodiments, the difference in refractiveindices between alternating layers is maximized in order to minimize thenumber of periods required to achieve a given reflectivity, and thethickness and refractive index of each period determines the stop bandand its limiting wavelength.

For some embodiments, distributed Bragg reflector (DBR) layers 305 athrough 305 z, and 306 a through 306 z includes a first DBR layercomposed of a plurality of p type Al_(x)Ga_(1-x)As layers, and a secondDBR layer disposed over the first DBR layer and composed of a pluralityof n or p type Al_(y)Ga_(1-y)As layers, where 0<x<1, 0<y<1, and y isgreater than x.

The scope of the compositional and optical difference in the structures305 and 306 will be described and specified in more detail subsequent tothe discussion of other embodiments.

In the illustrated example of FIG. 3A, the subcell B includes a highlydoped p-type aluminum gallium arsenide (“AlGaAs”) back surface field(“BSF”) layer 312, a p-type AlGaAs base layer 313, a highly doped n-typeindium gallium phosphide (“InGaP”) or AlGaAs emitter layer 314 and ahighly doped n-type indium gallium aluminum phosphide (“InGaAlP”) windowlayer 315. Other compositions may be used as well. The base layer 313 isformed over the BSF layer 312 after the BSF layer 312 is deposited overthe DBR layers 306.

The window layer 315 helps reduce the recombination loss and improvespassivation of the cell surface of the underlying junctions.

Before depositing the layers of the top cell A, heavily doped n-typeInGaP and p-type AlGaAs tunneling junction layers 316, 317 may bedeposited over the subcell B.

In the illustrated example, the top subcell A includes a highly dopedp-type indium aluminum phosphide (“InAlP₂”) BSF layer 318, a p-typeInGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320and a highly doped n-type InAlP₂ window layer 321.

A cap or contact layer 322 of GaAs is deposited over the window layer321 and the grid lines are formed via evaporation and lithographicallypatterned and deposited over the cap or contact layer 322.

Turning to another embodiment of the multijunction solar cell device ofthe present disclosure, FIG. 3B is a cross-sectional view of anembodiment of a four junction solar cell 4000 after several stages offabrication including the growth of certain semiconductor layers on thegrowth substrate up to the contact layer 322, with various layers andsubcells being similar to the structure described and depicted in FIG.3A.

The second embodiment depicted in FIG. 3B is similar to that of thefirst embodiment depicted in FIG. 3A except that an additional middlesubcell, subcell C, including layers 307 through 311 is now included,and since the other layers in FIG. 3B are substantially identical tothat of layers in FIG. 3A, the description of such layers will not berepeated here for brevity.

In the illustrated example of FIG. 3B, the subcell C includes a highlydoped p-type aluminum gallium arsenide (“AlGaAs”) back surface field(“BSF”) layer 307, a p-type InGaAs base layer 308 a, a highly dopedn-type indium gallium arsenide (“InGaAs”) emitter layer 308 b and ahighly doped n-type indium aluminum phosphide (“AlInP₂”) or indiumgallium phosphide (“GaInP”) window layer 309. The InGaAs base layer 308a of the subcell C can include, for example, approximately 1.5% In.Other compositions may be used as well. The base layer 308 a is formedover the BSF layer 307 after the BSF layer 307 is deposited over the DBRlayers 306.

The window layer 309 is deposited on the emitter layer 308 of thesubcell C. The window layer 309 in the subcell C also helps reduce therecombination loss and improves passivation of the cell surface of theunderlying junctions. Before depositing the layers of the subcell C,heavily doped n-type InGaP and p-type AlGaAs (or other suitablecompositions) tunneling junction layers 310, 311 may be deposited overthe subcell C.

Turning to another embodiment of the multijunction solar cell device ofthe present disclosure, FIG. 3C is a cross-sectional view of anembodiment of a four junction solar cell 600 after several stages offabrication including the growth of certain semiconductor layers on thegrowth substrate up to the contact layer 322, with various subcellsbeing similar to the structure described and depicted in FIG. 3B.

In a first embodiment of the present invention, shown in FIG. 3C, anintrinsic layer constituted by a strain-balanced multi-quantum wellstructure 500 is formed between base layer 410 b and emitter layer 411of middle subcell C. The strain-balanced quantum well structure 500includes a sequence of quantum well layers formed from alternatinglayers of compressively strained InGaAs and tensionally strained galliumarsenide phosphide (“GaAsP”). Strain-balanced quantum well structuresare known from the paper of Chao-Gang Lou et al., Current-EnhancedQuantum Well Solar Cells, Chinese Physics Letters, Vol. 23, No. 1(2006), and M. Mazzer et al., Progress in Quantum Well Solar Cells, ThinSolid Films, Volumes 511-512 (26 Jul. 2006).

In an alternative example, the strain-balanced quantum well structure500, comprising compressively strained InGaAs and tensionally strainedgallium arsenide, may be provided as either the base layer 410 b or theemitter layer 411.

In the illustrated example, the strain-balanced quantum well structure500 is formed in the depletion region of the middle subcell C and has atotal thickness of about 3 microns (mm). Different thicknesses may beused as well. Alternatively, as noted above, the middle subcell C canincorporate the strain-balanced quantum well structure 500 as either thebase layer 410 or the emitter layer 411 without an intervening layerbetween the base layer 410 b and emitter layer 411. A strain-balancedquantum well structure can include one or more quantum wells. Thequantum wells may be formed from alternating layers of compressivelystrained InGaAs and tensionally strained GaAsP. An individual quantumwell within the structure includes a well layer of InGaAs providedbetween two barrier layers of GaAsP, each having a wider energy band gapthan InGaAs. The InGaAs layer is compressively strained due to itslarger lattice constant with respect to the lattice constant of thesubstrate 400. The GaAsP layer is tensionally strained due to itssmaller lattice constant with respect to the substrate 400. The“strain-balanced” condition occurs when the average strain of thequantum well structure is approximately equal to zero. Strain-balancingensures that there is almost no stress in the quantum well structurewhen the multijunction solar cell layers are grown epitaxially. Theabsence of stress between layers can help prevent the formation ofdislocations in the crystal structure, which would otherwise negativelyaffect device performance. For example, the compressively strainedInGaAs well layers of the quantum well structure 500 may bestrain-balanced by the tensile strained GaAsP barrier layers.

The quantum well structure 500 may also be lattice matched to thesubstrate 400. In other words, the quantum well structure may possess anaverage lattice constant that is approximately equal to a latticeconstant of the substrate 400. In other embodiments, lattice matchingthe quantum well structure 500 to the substrate 400 may further reducethe formation of dislocations and improve device performance.Alternatively, the average lattice constant of the quantum wellstructure 500 may be designed so that it maintains the lattice constantof the parent material in the middle subcell C. For example, the quantumwell structure 500 may be fabricated to have an average lattice constantthat maintains the lattice constant of the AlGaAs BSF layer 410 a. Inthis way, dislocations are not introduced relative to the middle cell C.However, the overall device 600 is lattice mismatched if the latticeconstant of the middle cell C is not matched to the substrate 400. Thethickness and composition of each individual InGaAs or GaAsP layerwithin the quantum well structure 500 may be adjusted to achievestrain-balance and minimize the formation of crystal dislocations. Forexample, the InGaAs and GaAsP layers may be formed having respectivethicknesses about 100 to 300 angstroms. Between 100 and 300 totalInGaAs/GaAsP quantum wells may be formed in the strain-balanced quantumwell structure 500. More or fewer quantum wells may be used as well.Additionally, the concentration of indium in the InGaAs layers may varybetween 10 and 30%.

Furthermore, the quantum well structure 500 can extend the range ofwavelengths absorbed by the middle subcell C. An example of approximatequantum efficiency curves for the multijunction solar cell of FIG. 3C isillustrated in FIG. 7A. As shown in the example of FIG. 7A, theabsorption spectrum for the bottom subcell 603, subcell D, extendsbetween 890-1600 nm; the absorption spectrum of the middle subcell 602extends between 660-1000 nm, overlapping the absorption spectrum of thebottom subcell; and the absorption spectrum of the top subcell 601,subcell A, extends between 300-660 nm. Incident photons havingwavelengths located within the overlapping portion of the middle andbottom subcell absorption spectrums may be absorbed by the middlesubcell 602 prior to reaching the bottom subcell 603. As a result, thephotocurrent produced by middle subcell 602 may increase by taking someof the current that would otherwise be excess current in the bottomsubcell 603. In other words, the photo-generated current densityproduced by the middle subcell 602 may increase. Depending on the totalnumber of layers and thickness of each layer within the quantum wellstructure 500, the photo-generated current density of the middle subcell602 may be increased to match the photo-generated current density of thebottom subcell 603.

The overall current produced by the multijunction cell solar cell thenmay be raised by increasing the current produced by top subcell 601.Additional current can be produced by top subcell 601 by increasing thethickness of the p-type InGaAlP2 base layer 422 in that cell. Theincrease in thickness allows additional photons to be absorbed, whichresults in additional current generation. Preferably, for space or AM0applications, the increase in thickness of the top subcell 601 maintainsthe approximately 4 to 5% difference in current generation between thetop subcell A and middle subcell C. For AM1 or terrestrial applications,the current generation of the top cell and the middle cell may be chosento be equalized.

As a result, both the introduction of strain-balanced quantum wells inthe middle subcell 602 and the increase in thickness of top subcell Aprovide an increase in overall multijunction solar cell currentgeneration and enable an improvement in overall photon conversionefficiency. Furthermore, the increase in current may be achieved withoutsignificantly reducing the voltage across the multijunction solar cell.

In some embodiments, the sequence of first 501A and second 501Bdifferent semiconductor layers forms the base layer of the secondsubcell.

In some embodiments, the sequence of first and second differentsemiconductor layers comprises compressively strained and tensionallystrained layers, respectively.

In some embodiments, an average strain of the sequence of first andsecond different semiconductor layers is approximately equal to zero.

In some embodiments, each of the first and second semiconductor layersis approximately 100 nm to 300 angstroms thick.

In some embodiments, the first semiconductor layer comprises InGaAs andthe second semiconductor layer comprises GaAsP.

In some embodiments, a percentage of indium in each InGaAs layer is inthe range of 10 to 30%.

In some embodiments, the top subcell comprises InGaP and has a thicknessso that it generates approximately 4-5% less current than said firstcurrent.

FIG. 4A is a graph of the current density per unit energy versus thephoton energy and the wavelength of the incoming light in a solar cellunder the AM0 spectral environment.

FIG. 4B is a schematic representation of photons of differentwavelengths being absorbed by, or being transmitted through, differentsubcells in a three junction tandem solar cell with a single DBRstructure (DBR-1). In this particular representation the lightwavelengths λ_(C)>λ_(B)>λ_(A). The top subcell A is designed to absorblight of wavelength λ_(A), the middle subcell is designed to absorblight of wavelength λ_(B), and the bottom subcell is designed to absorblight of wavelength λ_(C).

FIG. 5A is a graph 500 of the reflectance of a single distributed Braggreflector structure as a function of wavelength such as known in theprior art. In this particular example, the wavelengths of light whichare most strongly reflected extend over the range 790 to 910 nm.Assuming that subcell B would absorb light in the wavelength range of790 to 910 nm, employment of this DBR structure below subcell B willreflect any light entering the DBR structure of that wavelength backinto subcell B, to allow photons of such wavelength to make a secondpass through subcell B, increasing the photocurrent of subcell B.

FIG. 5B is a graph 501 of the reflectance of a first distributed Braggreflector (DBR-2) structure according to the present disclosure comparedwith that of the graph 500 of FIG. 5A. In this particular example, thewavelengths of light which are most strongly reflected by DBR-2 extendover the range 850 to 920 nm. Assuming that subcell B would absorb lightin the wavelength range of 790 to 910 nm, employment of DBR-1 belowsubcell B will reflect any light entering DBR-1 of wavelength range 850to 920 nm back into subcell B to allow photons of such wavelength tomake a second pass through subcell B, increasing the photocurrent ofsubcell B.

FIG. 5C is a graph 502 of the reflectance of a second distributed Braggreflector (DBR-1) structure according to the present disclosure comparedwith that of graph 500 of FIG. 5A. In this particular example, thewavelengths of light which are most strongly reflected extend over therange 780 to 860 nm. Assuming that subcell B would absorb light in thewavelength range of 780 to 860 nm, employment of DBR-1 below subcell Bwill reflect any light entering DBR-1 of that wavelength range back intosubcell B to allow photons of such wavelength to make a second passthrough subcell B, increasing the photocurrent of subcell B.

The present disclosure contemplates the use of both DBR-2 and DBR-1 tomore finely tune or accurately cover the wavelength range to bereflected back into subcell B compared with the single DBR structure ofFIG. 5A.

FIG. 6 is a schematic representation of photons of different wavelengths(λ_(A), λ_(B1), λ_(B2), λ_(C)) being absorbed by, or being transmittedthrough, different subcells in a solar cell that includes twodistributed Bragg reflector (DBR) structures DBR-1 and DBR-2 accordingto the present disclosure.

FIG. 7A is a graph of the quantum efficiency versus wavelength in athree junction solar cell, represented by the top cell graph 601, themiddle cell graph 602, and the bottom cell graph 603.

FIG. 7B is a graph of the quantum efficiency versus wavelength in athree junction solar cell after incorporation of a structure in thesolar cell according to the present disclosure.

FIG. 8 is a graph of the doping profile in the base and emitter layersof a subcell in the solar cell according to the present disclosure. Insome embodiments, at least the base of at least one of the first A,second B or third C solar subcells has a graded doping, i.e., the levelof doping varies from one surface to the other throughout the thicknessof the base layer. In some embodiments, the gradation in doping isexponential. In some embodiments, the gradation in doping is incrementaland monotonic.

In some embodiments, the emitter of at least one of the first A, secondB or third C solar subcells also has a graded doping, i.e., the level ofdoping varies from one surface to the other throughout the thickness ofthe emitter layer. In some embodiments, the gradation in doping islinear or monotonically decreasing.

As a specific example, the doping profile of the emitter and base layersmay be illustrated in FIG. 8, which depicts the amount of doping in theemitter region and the base region of a subcell. N-type dopants includesilicon, selenium, sulfur, germanium or tin. P-type dopants includesilicon, zinc, chromium, or germanium.

In the example of FIG. 8, the emitter doping decreases from anywhere inthe range of approximately 5×10¹⁸ to 1×10¹⁷ free carriers per cubiccentimeter in the region immediately adjacent the adjoining layer toanywhere in the range of 1×10¹⁸ to 1×10¹⁵ free carriers per cubiccentimeter in the region adjacent to the p-n junction which is shown bythe dotted line in the referenced Figure.

The base doping increases from anywhere in the range of 1×10¹⁵ to 1×10¹⁸free carriers per cubic centimeter adjacent the p-n junction to anywherein the range of 1×10¹⁶ to 1×10¹⁹ free carriers per cubic centimeteradjacent to the adjoining layer at the rear of the base.

In some embodiments, the doping level throughout the thickness of thebase layer may be exponentially graded from the range of 1×10¹⁶ freecarriers per cubic centimeter to 1×10¹⁸ free carriers per cubiccentimeter, as represented by the curve 603 depicted in the Figure.

In some embodiments, the doping level throughout the thickness of theemitter layer may decline linearly from 5×10¹⁸ free carriers per cubiccentimeter to 5×10¹⁷ free carriers per cubic centimeter as representedby the curve 602 depicted in the Figure.

The absolute value of the collection field generated by an exponentialdoping gradient exp [−x/λ] is given by the constant electric field ofmagnitude E=kT/q(1/λ))(exp[−x_(b)/λ]), where k is the Boltzman constant,T is the absolute temperature in degrees Kelvin, q is the absolute valueof electronic change, and λ is a parameter characteristic of the dopingdecay.

The efficacy of an embodiment of the present disclosure has beendemonstrated in a test solar cell which incorporated an exponentialdoping profile in the three micron thick base layer of a subcell,according to one embodiment of the present disclosure. Followingmeasurements of the electrical parameters of the test cell, there wasobserved a 6.7% increase in current collection. The measurementsindicated an open circuit voltage (V_(oc)) equal to at least 3.014volts, a short circuit current density (J_(sc)) of at least 16.55mA/cm², and a fill factor (FF) of at least 0.86 at AM0.

The exponential doping profile taught by the present disclosure producesa constant field in the doped region. In the particular multijunctionsolar cell materials and structure of the present disclosure, the bottomsubcell has the smallest short circuit current among all the subcells.Since in a multijunction solar cell, the individual subcells are stackedand form a series circuit, the total current flow in the entire solarcell is therefore limited by the smallest current produced in any of thesubcells. Thus, by increasing the short circuit current in the bottomcell, the current more closely approximates that of the higher subcells,and the overall efficiency of the solar cell is increased as well. Inaddition to an increase in efficiency, the collection field created bythe exponential doping profile will enhance the radiation hardness ofthe solar cell, which is important for spacecraft applications.

Although the exponentially doped profile is the doping design which hasbeen implemented and verified, other doping profiles may give rise to alinear varying collection field which may offer yet other advantages,including for both minority carrier collection and for radiationhardness at the end-of-life (EOL) of the solar cell. Such other dopingprofiles in one or more base layers are within the scope of the presentdisclosure.

The doping profile depicted herein are merely illustrative, and othermore complex profiles may be utilized as would be apparent to thoseskilled in the art without departing from the scope of the presentinvention.

In some embodiments, the composition of the window layer is linearlygraded so that the concentration of Al in the window layer linearlyincreases from the bottom surface of the window layer to the top surfaceof the window layer.

In some embodiments, the window layer is composed of InAlP or InGaP andthe Al content at the bottom surface of the window layer is between 40.0and 48.5% by mole fraction.

In some embodiments, the composition of the window layer is graded sothat the lattice constant in the window layer is in compression at thebottom surface of the window layer, and increases to the top surface ofthe window layer so that the lattice constant in the window layer is incompression at the top surface.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

Although described embodiments of the present disclosure utilizes avertical stack of three subcells, various aspects and features of thepresent disclosure can apply to stacks with fewer or greater number ofsubcells, i.e. two junction cells, three junction cells, five, six,seven junction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell 309, with p-type and n-type InGaP is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or the emitter layer may alsobe intrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GalnAs, GaInPAs, AlGaAs, AlInAs, AlInPAs,GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GalnSb, AlGaInSb, AIN, GaN, InN,GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials,and still fall within the spirit of the present invention.

While the solar cell described in the present disclosure has beenillustrated and described as embodied in a conventional multijunctionsolar cell, it is not intended to be limited to the details shown, sinceit is also applicable to inverted metamorphic solar cells, and variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the foregoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

1. A method of forming a multijunction solar cell comprising: forming afirst solar subcell comprising an emitter layer and a base layercomposed of aluminum gallium arsenide or indium gallium arsenide, theemitter layer and the base layer forming a photoelectric junction;forming a second solar subcell disposed below the first solar subcelland comprising an emitter layer and a base layer forming a photoelectricjunction; and forming a combined DBR structure between the first solarsubcell and the second solar subcell with no intervening solar subcells,the combined DBR structure comprising a first distributed Braggreflector (DBR) structure and a second DBR structure; forming a firstdistributed Bragg reflector (DBR) structure disposed beneath the baselayer of the upper solar subcell and composed of a plurality ofalternating layers of different semiconductor materials withdiscontinuities in their respective indices of refraction and arrangedso that light can enter and pass through the upper solar subcell and atleast a first portion of which light in a first spectral wavelengthrange can be reflected back into the upper solar subcell by the DBRstructure, and a second portion of which light in a second spectralwavelength range can be transmitted through the DBR structure to thelayers disposed beneath the first DBR structure, wherein all if thewavelengths in the second spectral wavelength range is greater than allof the wavelengths in the first spectral wavelength range; and forming asecond distributed Bragg reflector (DBR) structure disposed beneath thefirst DBR structure and composed of a plurality of alternating layers ofdifferent semiconductor materials with discontinuities in theirrespective indices of refraction different from the layers of the firstDBR structure and arranged so that light can enter and pass through thefirst DBR structure and at least a portion of which light having asecond spectral wavelength range can be reflected back into the uppersolar subcell by the second DBR structure, and a third portion of whichlight in a third spectral wavelength range different from the first andthe second spectral wavelength ranges can be transmitted through thesecond DBR structure to the lower solar subcell disposed beneath thesecond DBR structure.
 2. The method of claim 1 further comprisingforming a metamorphic layer between the combined DBR structure and thesecond solar subcell.
 3. The method of claim 2 wherein for the secondsolar subcell, the emitter layer is composed of germanium and the baselayer is composed of germanium.
 4. The method of claim 1 furthercomprising forming tunnel diode layers between the combined DBRstructure and the second solar subcell.
 5. The method of claim 1 whereinforming the first solar subcell further comprises forming a windowlayer, tunnel diode layers, and a back surface field (“BSF”) layer,wherein the base layer is disposed on the BSF layer, the window layer isdisposed on the emitter layer, and the tunnel diode layers are disposedon the window layer.
 6. The method of claim 1 further comprising forminga first additional solar subcell above the first solar subcellcomprising forming an emitter layer and a base layer composed of indiumgallium aluminum phosphide, the emitter layer and the base layer forminga photoelectric junction.
 7. The method of claim 6 further comprisingforming a first tunnel diode layer, a second tunnel diode layer, and anucleation layer between the combined DBR structure and the second solarsubcell, wherein the nucleation layer is composed of indium galliumarsenide and is disposed on the emitter layer of the first additionalsolar subcell and the first tunnel diode layer composed of galliumarsenide is disposed on the nucleation layer and the second tunnel diodelayer composed of aluminum gallium arsenide is disposed on the firsttunnel diode layer, wherein the emitter layer of the first solar subcellis composed of indium gallium phosphide or aluminum gallium arsenide. 8.The method of claim 6, further comprising: forming a second additionalsolar subcell disposed between the first solar subcell and the firstadditional solar subcell, the second additional solar subcell comprisingan emitter layer composed of indium gallium arsenide or aluminum galliumarsenide and a base layer composed of aluminum gallium arsenide, whereinthe base layer of the first solar subcell comprises InGaAs.
 9. Themethod according to claim 6, further comprising forming a back surfacefield (“BSF”) layer composed of p-type aluminum gallium arsenidedisposed on the combined DBR structure and a window layer composed ofn-type indium gallium aluminum phosphide disposed on the emitter layerof the second solar subcell, wherein the base layer is disposed on theBSF layer.
 10. The method of claim 1, wherein one or more of the solarsubcells have a gradation in doping in the base layer that increasesapproximately exponentially from approximately 1×10¹⁵ free carriers percubic centimeter in a region adjacent the photoelectric junction toapproximately 4×10¹⁸ free carriers per cubic centimeter in a regionadjacent an adjoining layer and a gradation in doping in the emitterlayer that increases from approximately 5×10¹⁷ free carriers per cubiccentimeter in a region adjacent the photoelectric junction toapproximately 5×10¹⁸ free carriers per cubic centimeter in a regionimmediately adjacent an adjoining layer.
 11. The method of claim 1,wherein the emitter layer of the first solar subcell comprises highlydoped n-type indium gallium phosphide (“InGaP”).
 12. The method of claim1, wherein the half width value of reflection of the first DBR structureand the second DBR structure is in a range between 250 and 350 nm. 13.The method of claim 1, wherein the combined DBR structure includesforming alternating layers of lattice mismatched materials, the combinedDBR structure includes a first DBR layer composed of a plurality of ntype or p type Al_(x)Ga_(1-x)As layers, and a second DBR layer disposedover the first DBR layer and composed of a plurality of n or p typeAl_(y)Ga_(1-y)As layers, where 0<x<1, 0<y<1, and y is greater than x.14. The method of claim 1, wherein the combined DBR structure comprisesforming a sequence of alternating first and second differentsemiconductor layers, and wherein an average lattice constant of thesequence of alternating first and second semiconductor layers isapproximately equal to a lattice constant of a substrate.
 15. The methodof claim 1, wherein the first solar subcell comprises forming a highlydoped n-type indium gallium arsenide emitter layer and a highly dopedn-type indium gallium aluminum phosphide window layer.
 16. The method ofclaim 1, wherein the first solar subcell comprises a BSF layercomprising highly doped p-type aluminum gallium arsenide (“AlGaAs”). 17.The method of claim 1, wherein first spectral wavelength range—ofapproximately 780 to 860 nm.
 18. The method of claim 1, wherein thefirst solar subcell has a band gap in the range of 1.65 eV to 1.8 eV.19. The method of claim 1, wherein the first spectral wavelength rangeoverlaps the second spectral wavelength range by less than 10 nm. 20.The method of claim 1, wherein the first spectral wavelength range andthe second spectral wavelength range correspond to the spectralabsorption band of the first solar subcell.